1. Field of the Invention
The present invention relates to alkaline storage batteries and nickel positive electrodes in the alkaline storage batteries, and more particularly, to a paste-type nickel positive electrode suitable for a nickel-cadmium storage battery, a nickel-metal hydride storage battery, and a nickel-zinc storage battery and a method for producing the positive electrode thereof.
2. Description of the Related Art
Alkaline storage batteries such as nickel-cadmium storage batteries and nickel-metal hydride storage batteries are highly reliable and capable of being miniaturized. For these advantages, they are used both as the power sources of portable appliances and industrial power sources, depending on their sizes. In this type of alkaline storage batteries, sintered-type or paste-type nickel electrodes are used.
Generally, a paste-type nickel positive electrode is manufactured as follows. Firstly, a nickel hydroxide powder synthesized by mixing a sodium hydroxide aqueous solution and an aqueous solution containing a nickel salt as its main component. Thus obtained nickel hydroxide powder is mixed with a cobalt powder, a cadmium powder, or another such powder. Then, a binder and water are added to and mixed with the mixture to obtain a paste. Finally, the paste is applied to a porous substrate so as to obtain a nickel positive electrode. The paste-type nickel electrode thus manufactured has a higher energy density than the sintered-type nickel electrode.
The electric conductivity of nickel hydroxide acting as an active material is as low as about 10.sup.-14 S/cm when the valence of nickel is 2. However, it can be 10.sup.-2 S/cm or higher when the valence of nickel becomes larger through oxidation.
The electrochemical oxidation of nickel in nickel hydroxide from divalence to trivalence can be achieved with comparative ease. On the other hand, the electrochemical reduction from trivalence to divalence is very difficult because the electric conductivity of nickel hydroxide suddenly drops when the valence of nickel becomes 2.2 or lower. Consequently, the upper limit of the utilization rate of nickel hydroxide is about 80%.
In order to increase the utilization rate of nickel hydroxide, metallic cobalt or a cobalt compound such as cobalt hydroxide and cobalt oxide is added to the positive electrode, and as a result, the utilization rate is raised to nearly 100%. The reason for the increase in the utilization rate is that the metallic cobalt or the cobalt compound is converted during charge of a battery, through oxidation, into cobalt oxyhydroxide, which is conductive high-order oxide, forming a conductive network.
However, when the electric potential of the nickel positive electrode has fallen to 1.0 V or lower against the potential of the cadmium negative electrode or the hydrogen storage alloy negative electrode, the cobalt oxyhydroxide, which is conductive and in a high-order oxidation state, is converted into a cobalt compound having divalent cobalt, which is soluble in the electrolyte. Thus, when the voltage of an alkaline storage battery has decreased during a discharge process or a long-term preservation at a high temperature, the cobalt in the nickel positive electrode is either reduced in amount or localized. This reduction or localization of cobalt decreases the electric conductivity among nickel hydroxide active material particles, thereby deteriorating the utilization rate of the nickel hydroxide. In order to prevent the deterioration of the utilization rate, a conductive agent such as metallic nickel powder or a carbon powder which is stable in an alkaline electrolyte and has a high conductivity, may be added to the positive electrode. However, the use of these conductive agents has a problem that if the charge/discharge process of the battery is executed at a high temperature of 45.degree. C., the conductive agents are oxidized and gradually lose their conductivity.